If you send a persuasive message to a group, consider two important factors: the strength of your argument and the number of people who will process it.

For persuasive writing to be effective, an audience needs to be motivated to read the message (or hear it). And one factor that can affect a person’s motivation is how many other people are evaluating the same message. Social psychologists found that when a group is assigned to do a task, individuals within that group invest less effort on the task than when they are performing the work alone. This will depend on the size of the group. As size increases, individuals in the group are more likely to spend less effort on the assignment.

This tendency of people to invest less energy when others are available to pick up the slack is a phenomenon called social loafing, a term coined by psychologists Bibb Latane and John Darley.

Psychologist Richard Petty and colleagues wanted to know if social loafing applied not only to physical labor but also to mental tasks. So they conducted experiments to test whether people analyze an argument less extensively when they are in a group. In the experiments, the psychologists varied the number of people involved and they varied the strength of the persuasive arguments.

Experiment results

  • When the arguments were strong, groups that studied the message gave it a lower score because, with more people evaluating it, each person invested less mental effort. Individuals who studied the strong arguments rated it more highly because, reading it alone, they concentrated more and recognized the strengths of the writer’s position.
  • When the arguments were weak, groups assigned to evaluate the message gave it a higher rating because people in the group spent less time studying the message and therefore didn’t notice the flaws in the argument. Individuals gave the weak message a lower score because they expended the effort to read closely, and thus, they noticed the flawed arguments.

This has implications for messaging. Let’s say you need to propose a policy change, a new project, or an operational change. You would like to float the idea among some employees to gather early support and perhaps build momentum, so you want to write a persuasive message. If you have a strong argument, you might send the message only to a smaller group, initially, because each person is likely to spend more time processing the details.

Implications for juries

If you become a defendant in a court case and the evidence against you is weak, a small jury will probably be more beneficial to you, because fewer jurors are more likely to realize how flawed the evidence is, Petty wrote in the Journal of Personality and Social Psychology. If the evidence against you is strong, hope for a large jury.